The performance of an internal combustion engine obviously depends on the quality of the fuel the engine burns. This is particularly true for direct injecting internal combustion engines, e.g. for direct injecting diesel engines. In such engines, the fuel is ignited by injecting the fuel directly into the combustion chamber after the gas contained therein has been compressed, i.e. the fuel-mixture generations occurs within the combustion chamber.
One well-known fuel quality parameter is the cetane number in the case of diesel fuel. The cetane number measures the combustion quality of the fuel during compression ignition. The cetane number is related to a fuel's ignition delay, i.e. to the time period between the start of injection (SOI) of fuel into the combustion chamber and the start of combustion (SOC) of the fuel, i.e. of ignition. Diesel engines running a fuel with higher cetane fuels will show shorter ignition delay periods than a lower cetane fuels. Fuels with higher cetane number, thus, have short ignition delay time periods and provide more time for the fuel combustion process to be completed. Hence, the question of cetane number becames vital with high speeds of diesel engines.
In order to avoid an undesired performance of an engine, it is, therefore, known in the state of the art to determine a fuel quality parameter in particular for direct injecting internal combustion engines and restrict the range of load and speed which is admitted during operation of the engine accordingly, if low quality fuel was detected. Most known approaches, i.e. those known from WO2005/005813 A1 and WO2005/119034 A1 detect the fuel quality in fuel cut-off conditions of the engine by injecting small quantities of fuel and deriving a measure on the heat generated thereby through a careful evaluation of the engine's behavior. The state of the art aims at determining the cetane number of the fuel.
Although the cetane number may be helpful in limiting the operation range which is allowed for an engine running on a particular fuel, there is need of improvement to a more detailed control which not only limited the allowed speed and speed range and not restricted to certain operation conditions. This need is of great importance in connection with a reduction in pollutants in the exhaust gases of an engine, which reduction becomes a more and more pressing task in engine development.